Optical Polarization
for β-NMR at ISAC

In
conventional NMR experiments, nuclear spins are polarized by a large
static magnetic field. In the β-NMR apparatus at ISAC nuclear
polarization is produced in the ion beam in flight via optical pumping
with circularly polarized laser light. The polarizer relies on the well
known technique, developed for many years at CERN's ISOLDE, of
collinear
optical pumping to highly polarize the nuclear spins of 10–60 keV
radioactive beams. At ISAC, alkali-metal beams are longitudinally
polarized through optical pumping of a fast atomic beam, which is
created with up to 90% efficiency by charge exchange of the incident
ion
beam in a Na vapour jet (see Fig. 1). The
polarized
beam is then reionized in a cold He gas target with 66% efficiency and
directed to the experiments.

Fig. 1. Details of
the collinear optically pumped polarizer.

Reionizing the
beam gives some important advantages, enabling the use of electrostatic
optics to steer and focus the beam into several experiments. Two or
more
experiments may run simultaneously in the future by sharing the beam
using a fast kicker. The beam energy, and hence implantation depth, is
varied at the existing condensed matter experimental station from a few
hundred eV to 60 keV, by biasing the target. Post-polarization
electrostatic bends isolate the experiments from the Na vapour and
laser light in the polarizer. The polarization direction (longitudinal
or transverse with respect to the beam momentum) is determined by the
number of electrostatic 90° bends. A polarimeter measures the
transverse polarization of the ion beam after it has been
electrostatically deflected by 90°, and a second polarimeter is
being built to measure the longitudinal polarization of the undeflected
fast atomic beam. To date, 30 keV polarized beams of 8Li+
(over 107 ions s-1 at the experiment) and 9Li+
(105 ions s-1) have been produced at ISAC.

The 8Li
beam is polarized by optical pumping on the D1
transition with counter-propagating circularly polarized light. 8Li
has two ground state hyperfine levels, in common with most alkali-metal
isotopes, that in this case are split by 382 MHz (see Fig.
2). Both levels must be pumped in order to achieve high
polarization. The optical pumping laser is an argon-ion laser pumped,
standing wave Spectra-Physics 3900S Ti:sapphire (TiS) laser, specially
modified to lase at the two required frequencies at 673 nm. This is
very
near the short wavelength limit of TiS, and is achieved with a rear
cavity mirror centred at 650 nm and an R=99% output coupler.
The cavity length is shortened to increase the longitudinal mode
spacing to 381 MHz. A 1 mm thick, R=10% intra-cavity etalon
ensures operation on only two neighboring modes. The crystal gain
medium is in the centre of the cavity, where the nodes of one mode
coincide with the anti-nodes of the other, minimizing gain competition
and ensuring simultaneous oscillation of both modes. Typical laser
power
is ~100 mW on each mode. The linewidth of each mode is ~1 MHz on
timescales comparable to the 2 s transit
time of the atoms in the
optical pumping region. Vibrational instability causes frequency
fluctuations over a range of ±20 MHz on much longer timescales.
Doppler-shift tuning onto resonance is done by varying the deceleration
potential on the Na cell between 0 and 1 kV, thus scanning the beam
energy, while keeping the laser frequencies fixed. The tuning peak is
found by measuring the polarization at the polarimeter, or observing
the
laser induced fluorescence in the optical pumping region. The laser
frequencies are prevented from drifting more than ±3 MHz per day
by a system based on a frequency stabilized He–Ne laser.

Fig.
2. A scheme of the atomic levels of 8Li. The upper
part of the figure shows the hyperfine splittings in the ground state
and 1st excited state of 8Li, along with the pumping transitions. The
lower part shows the nearly degenerate Zeeman splitting of the
hyperfine
levels into 2F+1 magnetic sublevels m_F. Optical pumping with sigma+
light is shown, in which the only allowed absorptions are those having
an increase of angular momentum +1, Delta m_F = +1. Fluorescence can
occur on transitions satisfying Delta m_F = 0, +/-1. After many
cycles of optical pumping, all the atoms end up in the fully stretched
state m_F = 5/2, with full nuclear and electronic polarization, shown
by the red circle. Once in that state, they cannot be pumped out of it
by the laser light. The luorescence lifetime is 27 ns, which
allows for approximately 20 optical pumping cycles in the 2 us transit
time of atoms through the polarizer. Pumping with sigma- light pumps
all the atoms into the m_F = -5/2 ground state.

The key to
achieving high polarization is to match the optical pumping light
bandwidth to the energy (Doppler) broadening of the Li beam. The
typical
energy broadening is about 100 MHz, caused by multiple collisions with
Na atoms in the neutralizer, and cannot be avoided if high
neutralization efficiency is required. To overcome this problem, we use
resonant electro-optic modulators (EOMs) to produce laser sidebands and
broaden the effective laser bandwidth. Rate equation calculations
showed that a 19 MHz EOM and a 28 MHz EOM would work effectively in
series, in terms of both the total width and the sideband spacing of
about 10 MHz. Cesium vapour was tested as a neutralizer, since it was
thought that its smaller excitation energy than Na and greater mass
would produce less energy broadening. However, the energy broadening at
useful Cs densities was found to be larger than in Na vapour.

The Polarization
(reflected in the detected beta decay asymmetry) as the laser doublet
is
swept through the atomic resonance

Fig. 3. Typical tuning signal, showing β-decay
asymmetry
(uncorrected for different detector efficiencies) for both light
helicities.The statistical error bars are too small to show. The
variable offset is due to the beam shifting position on the foil.

For more
information can be found in Atsushi's ISAC Polarizer
Page. The idea of optical pumping is quite old, and it has been
applied in several ways, here is a brief bibliography: